Coating fluid for the preparation of a recording medium for use in inkjet printing

ABSTRACT

A coating fluid for forming a coating on a support for use in inkjet printing comprises a liquid medium having dispersed therein (a) an inorganic oxide selected from aluminium oxide and silica, (b) a binder polymer, and (c) a polymeric crosslinking agent containing functional groups for reaction with the inorganic oxide. The components (a), (b) and (c) are preferably dispersed in an aqueous liquid and the relative amounts of inorganic oxide to polymeric crosslinking agent are from 500:1 to 15:1 preferably 250:1 to 20:1, the relative amounts of inorganic oxide to binder polymer are from 50:1 to 2:1 preferably 20:1 to 4:1 and the relative amounts of binder polymer to polymeric crosslinking agent are from 40:1 to 2:1 preferably from 20:1 to 3:1 the amounts being by weight on a dry basis.

FIELD OF THE INVENTION

This invention relates to a coating fluid for the preparation of an inkabsorbing recording medium for use in inkjet printing, to an inkabsorbing material and to a process for the preparation of the inkabsorbing material.

BACKGROUND OF THE INVENTION

In recent years reflecting the use of electronic still cameras andcomputers, the hard copy technology to record images on paper sheets hasbeen rapidly developed. The goal is to bring the colour reproducibility,image density, gloss and weather resistance of the hard copy to the samelevel as silver halide photography. Ink jet systems have been used forhard copy recording since full colouring is easy and the printing noiseis low. In an ink jet system ink droplets are ejected at high speed fromnozzles to a recording material and the ink contains a large amount ofsolvent which is usually water.

The recording sheet for an inkjet printer is required to swiftly absorbthe ink and have good colour forming properties. To reduce the dry timeof the ink it has been previously proposed to provide a recording sheethaving a porous layer formed on a substrate, the porous layer beingformed of an inorganic oxides such as aluminium oxides or silica.

U.S. Pat. No. 4,517,244 discloses an ink jet recording material havingenhanced fixation rates comprising a support having therein a waterinsoluble resin having a water absorbing capacity of from 50 to 100times its own weight.

U.S. Pat. No. 5,212,008 discloses a recording sheet which comprises asubstrate having two coatings. The first coating comprises (a) acrosslinking agent and (b) a polymer such as a polysaccharide crosslinkable by (a). The second coating which is in contact with the firstcomprises a binder and compound such as an alkoxylated di-fattyquaternary compound.

European Patent Application No 916512 describes a coating fluid forapplication to a substrate for use in ink jet printing, the coatingfluid comprising alumina hydrate particles dispersed in an aqueousmedium which contains as binders a polyvinyl alcohol and a polymerisablecompound having a hydrocarbon group with a hydroxyl group or a compoundhaving a polyoxyalkylene chain, said compound having a substituent withan unsaturated bond or an epoxy group. The polymerisable compoundsdisclosed are monomers. The fluid is applied to a substrate such aspolyethylene terephthalate, dried and heated with a polymerizationinitiator to effect polymerization of the polymerizable compound.

Compared with the use of polyvinyl alcohol without the polymerizablebinder, the advantage stated is that peeling of the coated layer isavoided and good image quality is maintained for a long period of time.

PROBLEM TO BE SOLVED BY THE INVENTION

There is a continuing need to improve the ability of the coating toabsorb ink applied to it since this reduces the drying time.

The present invention provides a solution to this problem by including across linking agent capable of reacting with the aluminium oxide orother inorganic oxide in the coating fluid to form a more rigidstructure and increase the rate of drying of ink applied to the coating.

SUMMARY OF THE INVENTION

According to the present invention there is provided a coating fluid forforming a coating on a support for use in inkjet printing said fluidcomprising an liquid medium having dispersed therein (a) an inorganicoxide selected from aluminium oxide and silica, (b) a binder polymer,and (c) a polymeric cross linking agent containing functional groups forreaction with the inorganic oxide.

The components (a), (b) and (c) are preferably dispersed in an aqueousliquid.

According to another aspect of the present invention there is provided aprocess for the preparation of a coated support for use in inkjetprinting which process comprises applying to the support a coating fluidas hereinbefore defined and causing the crosslinking agent to react withthe inorganic oxide.

According to a further aspect of the present invention there is provideda support having a coating formed from a coating fluid as hereinbeforedefined.

ADVANTAGEOUS EFFECT OF THE INVENTION

The advantage of the present invention is that the crosslinking of theinorganic oxide creates a more rigid structure and has the effect thatink applied to the coating dries more quickly. The cross linking agentis a preformed polymer and has a sufficient number of functional groupsto increase the rigidity of the structure but not so many functionalgroups that it increases the rigidity too much which would make thecoating brittle and risk cracking of the coating or so many functionalgroups that crosslinking occurs to an undesirably high degree such thatthe absorptive capacity of the layer is unduly reduced through excessiveconstraint upon its swelling. The result is that the coating retainssufficient flexibility to avoid cracking under conditions of normal use.

DETAILED DESCRIPTION OF THE INVENTION

The relative amounts of inorganic oxide to polymeric cross linking agentare conveniently from 500:1 to 15:1 preferably 250:1 to 20:1, therelative amounts of inorganic oxide to binder polymer are convenientlyfrom 50:1 to 2:1 preferably 20:1 to 4:1 and the relative amounts ofbinder polymer to polymeric cross linking agent are conveniently from40:1 to 2:1 preferably from 20:1 to 3:1 the amounts being by weight on adry basis.

The binder polymer may be any of those known in the art. Such polymersinclude polyvinyl pyrrolidone, polyvinyl acetate, copolymers of styreneand acrylates, copolymers of styrene and butadiene, polyacrylates andpolymethacrylates, copolymers of acrylic and methacrylic acid,polyacylamides and polymethacrylamides.

It is desirable that the binder polymer is soluble or dispersible in themedium which is usually water. Conveniently the polymer will form acolloidal dispersion in water.

The preferred binder polymer is polyvinyl alcohol, and it is preferredto use one having a molecular weight of from 1,000 to 100,000,preferably from 5,000 to 50,000. The degree of hydrolysis may be atleast 80% preferably at least 90% or 95%.

It is desirable that the inorganic oxide is dispersible in the mediumwhich is usually water.

Preferably the inorganic oxide is capable of forming a colloidaldispersion in water. When the inorganic oxide is aluminium oxide itusually comprises aluminium oxide hydrate particles. These are capableof being dispersed to form a colloidal dispersion in water.

The aluminium oxide is preferably fumed or colloidal since the particlesare smaller (less than 100 nm) although other forms such as precipitatedor gel may be used.

As the aluminium oxide hydrate one represented by the compositionalformula Al₂O₃.nH₂O where n is from 1 to 1.5 may be used. It is suitableas an ink receiving layer since the colourant is well absorbed and fixedto it. Aluminium oxide in the form of Bohemite or pseudo-Bohemite may beused. The latter is described in U.S. Pat. No. 5,104,730.

The crosslinking agent is a molecule having at least two functionalgroups capable of reacting with —AlOH or —SiOH sites in the inorganicoxide. The functional groups are preferably silanol or silanol-forminggroups such as SiOR where R is an alkyl group containing up to 6 carbonatoms or other monovalent hydrocarbon group containing from 1 to 6carbon atoms. Alternatively the functional groups may be carboxylicacid, epoxy of epoxy-forming groups.

Desirably the molecular weight of the cross linking agent is such thatit is soluble or dispersible to form a colloidal dispersion in water.The molecular weight is not critical but will typically be in the rangefrom about 1000 to about 30,000 for example from about 1500 to about10,000.

The polymeric crosslinking agent may have the structure:polymer-----A------Si(OR)₃ where

-   -   A is optional and when present is a linking group containing not        more than 12 carbon atoms, R is hydrogen or a monovalent        hydrocarbon group containing from 1 to 6 carbon atoms and the        polymer is selected so that the crosslinking agent is        dispersible in water

A preferred group of crosslinking agents are those containing waterdispersible protein moiety linked through a short chain of carbon atomstypically 2 to 12 carbon atoms to a silicon atom containing thefunctional groups for the cross linking.

A typical member of the group is Crodasone C obtained commercially fromCroda Chemicals limited which has the structure:gelatin-CH₂CH(OH)—O—(CH₂)₃—Si(OH)₃

-   -   where the gelatin has a molecular weigh in the range from about        2,000 to 5,000. The gelatin has been partially hydrolysed to        render it water dispersible and reduce its viscosity.

This molecule crosslinks through the silanol groups slowly on drying at20 degrees Centigrade and fast at 60 degrees Centigrade.

Although it is beneficial to make the coating more rigid it is desirablethe coating retain a degree of flexibility.

It is preferable therefore that the crosslinking agent has not more thanone functional group per 400 units of molecular weight, more preferably1000 units of molecular weight.

As the means to coat the coating fluid of the present invention onto thesubstrate e.g. a dye coater, a roll coater, an airknife coater, a bladecoater, a rod coater or a comma coater may be employed.

The crosslinking of the binder polymer by the cross linking agent may beeffected at room temperature e.g. 20 degrees Centigrade, or may bepromoted by beating, ultraviolet rays or an electron beam after coating.

The coating when dry forms a porous layer which may have pores havingpore radii which are from 1 to 15 nm, and the pore volume may be from0.4 to 2.0 cc/g. The porous layer will then have an adequateabsorptivity, and transparency.

The pore radii may be from 3 to 10 nm. The pore radii distribution maybe measured by a nitrogen adsorption/desorption method.

The thickness of the porous layer on the recording sheet is suitablyselected depending upon eg the specification of the printer. Thethickness may be above or below 100 micrometres. It is frequently in therange from about 30 to about 50 micrometres at a void fraction of 0.5.(ie volume of pores is 50% of the layer)

The invention is illustrated by the following Examples.

Functional Polymers to Serve as cross linking agents for Ink Jet Media

EXAMPLE 1

A coating fluid was prepared by mixing together at room temperature anaqueous aluminium oxide slurry containing about 40% by weight of solids,a dispersion of polyvinyl alcohol (98% hydrolysis, molecular weight13,000 to 23,000) in water containing about 10% by weight of solids anda dispersion of the polymeric crosslinking agent containing about 2.5%by weight of solids. The amounts of the three dispersions were adjustedto yield a coating containing the three components in the weight ratioof about 90:9:1 aluminium oxide: polyvinyl alcohol: polymeric crosslinking agent on a dry basis. After mixing the dispersions together themixture was used immediately to coat an ESTAR™ support to form an inkabsorbing layer

Using a rod coater the ink absorbing layer was coated at a lay down ofbetween 24 g/m² and 28 g/m² of aluminium oxide slurry, between 2.5 and3.5 g/m² polyvinyl alcohol and a cross linking agent at a laydown ofapprox. 0.25 g/m². The aluminium oxide, PVA and cross linking agent weretherefore coated at a ratio of approximately 90:9:1 by weight. on a drybasis.

The coating was allowed to dry at 20 degrees centigrade. Thecrosslinking reactions were allowed to take place during the drying. Noheating or other means to assist the cross linking was necessary.

The function of the crossslinking agent in the ink absorbing layer is tocrosslink with the inorganic oxide, which creates a more rigidstructure. This forces the aluminium oxide particles to be moreseparated, which in turn imparts more free volume. This can be evaluatedby the measuring the density of ink transferred to a piece of plainpaper, which had been sandwiched to the printed image immediately afterthe sample exited the ink jet printer. The lower the density of ink onthe plain paper, the faster the sample dried. An alternative method isto visually assess the piece of plain paper that had been sandwiched tothe printed image and identify the point at which the ink transferdisappears. Knowing the time taken for the image to print, this can thenbe used to calculate the average time taken for the sample to dry.

The results in table 1 show the density of ink transferred to the nieceof plain paper 60 seconds after the sample has exited the printer.Average time taken for the samples to dry is also shown compared to thecontrol coating where no crosslinking agent was added. TABLE 1 EpsonStylus Photo 700 Canon BJC4400 AOS PVA Glue 1 Glue 2 Ink Density Avg DryInk Density Avg Dry (g/m²) (g/m²) (g/m²) (g/m²) @ 60s Time (Mins) @ 60sTime (Mins) 24.42 2.71 0.244 — 0.001 0.42 0.760 2.17 24.97 2.77 — 0.2500.062 0.85 0.080 0.50 27.20 3.06 — — 1.211 >4.27 0.859 3.33 Time TakenTo Print Image (Mins) 4.27 5.95Key:AOS = Aluminium Oxide Slurrycrosslinking agent 1 = Crodasone* Sesame (sesame amino acid polysiloxanecopolymer) referred to as glue 1 in the above tablecrosslinking agent 2 = Crodasone* W (aqua and hydrolysed wheat proteinhydroxpropyl polysiloxane) referred to as glue 2 in the above table.*obtained commercially from Croda Colloids Limited of the U.K.60 degrees Centigrade.

The results show quite clearly for the two ink sets tested, that boththe average time taken to dry the sample and the density of inktransferred 60 seconds after the sample was ejected form the printer areimproved when the crosslinking agent is present in the formulation.

Printer set-ups:

Epson Stylus Photo 700 printer was used with the following settings:

-   -   Print quality—superfine 1440 dpi    -   Photo quality ink jet paper

Canon BJC4400 printer was used with the following settings:

-   -   High print quality    -   Photo colour    -   Automatic print model    -   Glossy photo paper

1-9. (cancelled).
 10. A coating fluid for forming a coating on a supportfor use in inkjet printing, said fluid comprising a liquid medium havingdispersed therein (a) an inorganic oxide selected from the groupconsisting of aluminium oxide and silica, (b) a binder polymer, and (c)a polymeric crosslinking agent containing functional groups for reactionwith the inorganic oxide, wherein the polymeric crosslinking agent hasnot more than one functional group for reaction with the inorganic oxideper 400 units of molecular weight.
 11. A coating fluid according toclaim 10, wherein the polymeric crosslinking agent has not more than onefunctional group for reaction with the inorganic oxide per 1000 units ofmolecular weight.
 12. A coating fluid according to claim 10 wherein thecomponents (a), (b) and (c) are dispersed in an aqueous liquid.
 13. Acoating fluid according to claim 10 wherein the binder polymer ispolyvinyl alcohol.
 14. A coating fluid according to claim 10, whereinthe relative amount of inorganic oxide to polymeric crosslinking agentis from 500:1 to 15:1, the amount being by weight on a dry weight basis.15. A coating fluid according to claim 10, wherein the relative amountof inorganic oxide to polymeric crosslinking agent is from 250:1 to20:1, the amount being by weight on a dry weight basis.
 16. A coatingfluid according to claim 10, wherein the relative amount of inorganicoxide to binder polymer is from 50:1 to 2:1, the amount being by weighton a dry weight basis.
 17. A coating fluid according to claim 10,wherein the relative amount of inorganic oxide to binder polymer is from20:1 to 4:1, the amount being by weight on a dry weight basis.
 18. Acoating fluid according to claim 10, wherein the relative amount ofbinder polymer to polymeric crosslinking agent is from 40:1 to 2:1, theamount being by weight on a dry weight basis.
 19. A coating fluidaccording to claim 10, wherein the relative amount of binder polymer topolymeric crosslinking agent is from 20:1 to 3:1, the amount being byweight on a dry weight basis.
 20. A coating fluid according to claim 10,wherein the polymeric crosslinking agent has the structure:polymer-----A------Si(OR)₃ where A is optional and when present is alinking group containing not more than 12 carbon atoms, R is hydrogen ora monovalent hydrocarbon group containing from 1 to 6 carbon atoms andthe polymer is selected so that thecrosslinking agent is dispersible inwater.
 21. A coating fluid as claimed in claim 10, wherein the polymericcrosslinking agent has been obtained by the partial hydrolysis of aprotein to render it water dispersible and where the protein has asilane containing functional groups for the cross linking of theinorganic oxide.
 22. A coating fluid as claimed in claim 10, wherein thepolymeric crosslinking agent has the structure:gelatin-CH₂CH(OH)—O—(CH₂)₃—Si(OH)₃ where the gelatin has a molecularweight in the range from about 2,000 to about 5,000 and has beenpartially hydrolysed.
 23. A coating fluid for forming a coating on asupport for use in inkjet printing, said fluid comprising a liquidmedium having dispersed therein (a) an inorganic oxide selected from thegroup consisting of aluminium oxide and silica, (b) polyvinyl alcohol asa binder polymer, and (c) a polymeric crosslinking agent containingfunctional groups for reaction with the inorganic oxide, which polymericcrosslinking agent has the structure:polymer-----A------Si(OR)₃ where A is optional and when present is alinking group containing not more than 12 carbon atoms, R is hydrogen ora monovalent hydrocarbon group containing from 1 to 6 carbon atoms andthe polymer is selected so that the crosslinking agent is dispersible inwater, and wherein the polymeric crosslinking agent has not more thanone functional group for reaction with the inorganic oxide per 1000units of molecular weight.
 24. A process for the preparation of a coatedsupport for use as an ink absorbing substrate in inkjet printing whichprocess comprises applying to the support a coating fluid as claimed inclaim 10 and causing the crosslinking agent to react with the inorganicoxide.
 25. A coated support for use in ink jet printing said supporthaving a coating formed by the application to the support of a coatingfluid according to claim 10.